Low power consumption and high energy efficiency power supply MCU solution

　　When batteries need to power a product for years or even decades, continuous improvement of MCU integration products and slight modification of basic processor architectures cannot meet the rapidly increasing energy-saving needs. For many energy-sensitive products, such as meters, building automation products, security products, and portable medical devices, if there is a conflict between energy-saving needs and processing power, large-scale development of MCU design is necessary.

　　EnergyMicro has taken a 'bluesky' approach to designing its low-power EFM32Gecko microprocessor and has developed software and hardware tools to support the product (Figure 1). EnergyMicro has now produced a device that consumes only one-quarter of the energy consumed by existing 8-bit, 16-bit and 32-bit MCUs, significantly extending the life of existing batteries. In other words, with such an energy-saving MCU, product designers can significantly reduce the cost and size of batteries. And for some products, such as energy meters and security equipment, with maintenance annotations on frequency, cost and carbon footprint, batteries will need to be replaced even less frequently.

　　Getting such low power qualifications on an MCU is no easy feat and requires years of development and real innovation. Check out the EnergyMicro website for the top 10 reasons why the 32-bit EFM32 is the world's most energy-efficient microcontroller, and there are certainly many more reasons.

　　Let's put the "ultra-low energy" specmanship aside for now. When the battery charge is limited, how well the MCU can use energy over time becomes important. Reducing the energy consumption and time during the product's dormant period is as important as the work to be done during the active period. The EFM32 MCU is based on the ARM Cortex-M3 processing core and is designed to greatly reduce the power consumption in active mode. In benchmark tests, the 32MHz EFM32 actually requires a 3V supply to run correct Flash code at 180μA/MHz.

　　That’s all well and good, but how long the MCU takes to process tasks also has a major impact on energy savings. So, using a 32-bit Cortex-M3 that is more efficient than 8-bit and 16-bit devices, and can execute tasks in much shorter clock cycles, can significantly reduce the active period of the product. By keeping the active period as short as possible, 32-bit MCUs spend more time in deep sleep mode. People forget that 32-bit processors couldn’t deliver sub-µA standby modes in the past, but with the right low-power design techniques, it can now be done. The EFM32 can provide all the baseline functions such as real-time counters, RAM and CPU retention, brownout detection and power-on reset in deep sleep mode, all using only 0.9μA of energy.

　　Typically, in the target applications we mentioned, the MCU duty cycle can be very short and the MCU can stay in deep sleep state up to 99% of the time. Therefore, the consumption here is really important for the overall energy saving.

　　If the MCU takes a long time to wake up the product from deep sleep and re-enter active mode, its advantages will be lost. Why? Because when the MCU enters the active state from deep sleep, there is always a wake-up cycle during which the processor must wait for the oscillator and power system to stabilize before it can start executing code. Since no processing can be performed during this period, the energy consumed by wake-up is wasted, so shortening the wake-up time is important to reduce overall energy consumption.

　　Not only that, but MCU applications also impact real-time requirements, which usually means that wake-up times must be kept to a minimum so that the MCU can respond to an event within a certain period of time. Since many applications require longer latencies than the wake-up times of many existing MCUs, devices often cannot fully enter deep sleep - which is not a good solution for energy-saving applications.

　　To solve this problem, EFM32 uses various design techniques to reduce the wake-up time from deep sleep to just 2μs, ensuring that the CPU uses the least amount of energy when it starts processing tasks.

　　To fully control and truly optimize energy savings, system designers need the flexibility to choose from a few well-structured energy modes. As shown in Table 1, the EFM32 provides several modes that allow designers to use resources at any point in time to maximize energy efficiency.

　　Even though these energy modes may seem a bit crude to some observers, enabling or disabling different peripherals allows for more fine-tuning of resources in each mode. Either way, the EFM32's energy modes help eliminate any waste of energy.

　　Of course, the peripheral function blocks provided by low-power MCUs need to be deliberately designed for low-power operation, and the EFM32 is no exception. For example, the MCUsport's 8-channel 12-bit ADC uses 350µA and a conversion rate of 1Msamples/1 second at full resolution; a 4×4-segment LCD controller uses only 550nA sporting integrated voltage enhancement, contrast, animation and flashing functions; and a special low-energy UART and a full UART with a 32kHz clock consume only 150nA at a data transmission speed of 9600 baud.

　　To achieve better power saving, an important innovation was to create an MCU architecture that allows the CPU to automatically retain peripheral functions. Therefore, the EFM32 peripherals are designed to take care of themselves, either letting the CPU handle other high-level tasks or simply going to sleep, both of which can save energy.

　　To take automation a step further, the EFM32 introduces another programmable interconnect structure, called the peripheral reflex system, into an MCU architecture (Figure 2), which enables communication between peripherals without CPU intervention, thereby further reducing energy consumption.

　　Having an ultra-powerful MCU in itself does not guarantee the user the lowest energy consumption. Having the right tools to identify and prevent energy consumption early in the product prototyping process can significantly reduce the overall energy consumption of the final product.

　　At the Electronica 2010 show, EnergyMicro announced the upcoming development of SimplicityStudio, a complete graphical user interface development kit for EFM32 microcontrollers. It will provide faster access to all the information, files and tools needed by hardware, firmware and software engineers to effectively develop embedded systems. Most of these tools are available in stock.

　　The EFM32 development kit has an AdvancedEnergyMonitoring (AEM) system that continuously measures the current consumed. This measurement is complete and accurately depicts the power used over time, allowing real-world applications to be optimized for low-power operation (Figure 3).

　　When using the energyAwareProfiler "energy debugging" software tool, AEM allows users to instantly identify the actual source code that is executed at a given time as shown on the energy graph. This immediately points out to engineers the parts of the program that are generating high energy consumption, allowing the code to be optimized and energy savings to be managed more closely.